Time ‒ Hourglass, Clepsydra (Water Clock)

The **hourglass** is used to measure the time of sand movement caused by the Earth's gravitation. They consist of two glass flasks placed above each other and connected by a narrow neck. The sand from the top flask is gradually poured into the bottom flask. After the measured time period has elapsed, the upper bulb is empty and the clock has to be reversed so that a further period can be measured.

The **water clock** - *the clepsydra* - consist of a container with a small hole through which water flows slowly out. Water leakage is caused by the Earth's gravitation. The markings inside the container indicate the elapsed time. Once the measured time has elapsed, it is necessary to refill the container with water so that a further period of time can be measured.